A study has been conducted of the feasibility, costs, and emissions benefits of a number of alternative-fuel and conventional technologies for controlling emissions from Diesel transit buses in the city of Santiago, Chile. Alternative fuels considered were methanol, compressed natural gas (CNG), and propane - using a variety of engine technologies. These were compared with each other and with conventional emission control approaches - an existing inspection/maintenance program, particulate trap-oxidizers, and low-emission Diesel engines. Cost estimates were developed for both new buses and retrofit of existing buses. Use of a consistent analytical framework and assumptions across all of the technologies allowed realistic comparisons between them. Overall, trapoxidizers and compressed natural gas in a lean-burn, converted Diesel engine provided the best combinations of cost and emissions reduction. Proper maintenance was also found to be a critical factor in reducing emissions - strengthening the existing inspection/maintenance program was recommended as a first priority.Diesel buses form a vital component of the transportation system in most cities of the developing world. The large numbers and high emissions levels of these buses make them major contributors to the poor air quality found in many urban areas of less-developed nations. As urban populations and air pollution increase, governments are increasingly seeking solutions to the problem of bus emissions. Use of alternative “clean” fuels such as methanol, ethanol, propane, and especially natural gas is frequently considered as an emissions control strategy. This is especially true when (as is often the case) these fuels could be derived from domestic sources, thus reducing dependence on imported oil or increasing the amount of domestically produced oil available for export. Other, more “conventional” emissions control strategies for Diesel buses include improved maintenance, inspection/maintenance programs, trap-oxidizers, and low-emission Diesel engines.Santiago, the capital and principal city of Chile, suffers from severe air pollution, much of which is attributed to the 11, 500 Diesel buses which constitute the dominant mode of transportation. In an effort to address this problem, the Gomision Nacional de Energia (National Energy Commission) contracted with Sierra Research to evaluate the feasibility, costs, and emissions benefits of alternative fuel technologies and other approaches to controlling Diesel bus emissions (1)*. This evaluation served as input to a policy study by the Comision staff (2). That study included other technologies and fuels, with somewhat different assumptions concerning long and short-term fuel prices and economic conditions.The primary emphasis of the present study was on retrofit of existing vehicles, although new-vehicle technologies were examined as well. The following were some of the specific topics examined: compressed natural gas (CNG) fuel; propane fuel; methanol fuel; trap-oxidizers; and emissions standards for new Diesel engines.A number of specific engine technologies were considered for use with each alternative fuel. In addition, the effectiveness of the existing heavy-duty Diesel inspection/maintenance program was evaluated, and suggestions were developed for improving its effectiveness. To ensure that the study's conclusions were not distorted by differences in assumptions and analytical procedures, a consistent set of assumptions and analytical framework were used for each evaluation.The costs and other results developed in this study are strictly applicable only to the Chilean case. Because Chile's general situation is similar to that of many other developing countries, however, the general conclusions reached may be more broadly applicable. In addition, the methodology and approach used may serve as an example and a starting point for other analyses.